The present invention relates to an image recording apparatus employing an electrophotographic system.
Conventionally, there is known an image recording device utilizing an electrophotographic system in which a surface of a photoconductive drum is exposed to light to form a latent image on the drum surface. Toner is then applied to the latent image to develop the image, and the developed image is transferred onto a recording sheet and is fixed by a fixing unit. Such image recording device is chiefly employed in a copying machine. In recent years, however, the image recording device is being utilized as a printer for printing output from a computer, and is formed as is a laser beam printer.
The laser beam printer comprises, for example as illustrated in FIG. 1, a photoconductive drum 1. Arranged about the photoconductive drum 1 in order in a rotational direction thereof are a charging station A, an exposure station B, a developing station C, a transferring station D, a toner-cleaning station E, and a discharge station F.
The arrangement is such that at the exposure station B, the laser beam scans the surface of the drum 1 which has been uniformly charged at the charging station A, to thereby form a latent image on the charged drum surface. Toner is then applied at the developing station C to the latent image to develop the same. Subsequently, the developed toner image is transferred at the transferring station D onto the recording sheet P fed at a velocity identical with the circumferential speed of the photoconductive drum 1. The recording sheet P carrying the toner image transferred thereon at the transfer station D is guided and/or fed by guide rollers to a fixing station G. The recording paper P is then heated and/or pressed at the fixing station G so that the toner image will be fixed on the surface of the recording sheet P.
The constitution of the laser beam printer is substantially similar to that of an electronic copying machine except for the exposure station. Actually, principal parts of the electronic copying machine are often commonly used for conventional laser beam printers.
The electronic copying machine is designed so that an image-carrying surface can instantly be viewed by transferring the toner image onto the upper surface of the recording sheet and discharging the recording sheet with the image-carrying side up (so-called "faceup discharging"). This naturally results in the fact that the faceup discharging system is employed in the laser beam printer having the principal parts in common with the electronic copying machine.
However, printers are normally used to print out the sheets continuously and, if the sheets are discharged with the image-carrying surfaces up, they will be stacked in inverted order of pages. As a result, it has been desirable for the laser beam printers capable of discharging the sheets with their image-carrying surfaces down (so-called "facedown discharging") so the the sheets are stacked in the proper order of pages.
Although such a printer is not limited in constitution to what has been described above, each of the operating units disposed around a photoconductive drum has to be detachable not only for maintenance but also for replacement after the passage of its life is taken into consideration. For this reason, these operating units have to be provided with respective locking mechanisms to make them detachable. Moreover, the printer also has to be arranged so that it is not operated while any one of the detachable operating units is not set in position so that it is not operated without it. As a result, the printer may become complicated in construction and costly because a number of locking mechanisms are required therefor.